At present, the majority of silicon single crystals used for manufacturing a substrate for forming a circuit component of a LSI (large scale integrated circuit) and the like have been pulled by the CZ method. FIG. 1 is a diagrammatic sectional view of a conventional apparatus for pulling a single crystal using the CZ method. In the figure, reference numeral 21 represents a crucible.
The crucible 21 comprises a bottomed cylindrical quartz crucible 21a and a bottomed cylindrical graphite crucible 21b fitted on the outer side of the quartz crucible 21a. The graphite crucible 21b is fitted in a graphite fitting part 28a. The crucible 21 and the fitting part 28a are supported with a support shaft 28 which rotates in the direction shown by the arrow A in the figure at a prescribed speed. A heater 22 of a resistance heating type and a heat insulating mold 27 arranged around the heater 22 are concentrically arranged around the crucible 21. The crucible 21 is charged with a melt 23 of a material for forming a crystal which is melted by the heater 22. On the central axis of the crucible 21, a pulling axis 24 made of a pulling rod or wire is suspended, and at the lower end thereof, a seed crystal 35 is held by a holder 34a. These parts are arranged within a water cooled type chamber 39 wherein pressure can be controlled, while an optical measuring means 19 is arranged above the crucible 21 outside the chamber 39.
In pulling a single crystal 36 using the above-mentioned apparatus for pulling a single crystal 30, the pressure in the chamber 39 is reduced and an inert gas is introduced into the chamber 39 so as to make an inert gas atmosphere under reduced pressure, which is maintained for a period of time. Then, the material for forming a crystal is melted by the heater 22.
While the pulling axis 24 is rotated on the same axis in the reverse direction of the support shaft 28 at a prescribed speed, the seed crystal 35 held by the holder 34a descends and is brought into contact with the melt 23, and the pulling of the single crystal 36 from the melt 23 is started. When the crystal is made to grow at the lower end portion of the seed crystal 35 as the pulling is carried out, the crystal is once narrowed down to have a prescribed diameter, leading to the formation of a neck 36a (hereinafter, referred to as the necking step).
After forming a shoulder 36b by making the neck 36a grow to have a prescribed diameter, a main body 36c having a uniform diameter and a prescribed length is formed. The diameter of the single crystal 36 is gradually decreased and the temperature of the whole single crystal 36 is gradually lowered, leading to the formation of an end-cone. Then, the single crystal 36 is separated from the melt 23. Here, in the above steps, the diameter of the single crystal 36 is measured by measuring luminance of the growth interface (fusion ring) on the surface of the melt 23 when the single crystal 36 is formed, using the optical measuring means 19, and is controlled on the basis of the results.
In the above conventional method for pulling a single crystal, the neck 36a is formed under the seed crystal 35 in order to exclude the dislocation induced by a thermal shock when the seed crystal 35 is brought into contact with the melt 23. Ordinarily, the neck 36a has a diameter of 3 mm or so and a length of 30 mm or so. In pulling a single crystal 36 having a diameter of about 6 inches and a weight of 80 kg or so, even the neck 36a having the above diameter can sufficiently support the pulled single crystal 36.
Recently, however, in order to manufacture a more highly integrated semiconductor device at a lower cost and more efficiently, the wafer has been required to have a larger diameter. Now, for example, the production of the single crystal 36 having a diameter of about 12 inches (300 mm) and a weight of 300 kg or so is desired. In this case, the neck 36a having a conventional diameter(usually 3 mm or so) cannot withstand the weight of the pulled single crystal 36 and breaks, resulting in the failing of the single crystal 36.
In order to solve the above problem, a method for pulling a single crystal has been disclosed. The diameter of the single crystal is gradually enlarged once under the neck to form an enlarged portion. The diameter of the crystal is gradually reduced to form a reduced portion having a larger diameter than the neck, which is supported by a supporting apparatus when the single crystal is pulled.
FIG. 2 is a diagrammatic sectional view of a conventional apparatus for pulling a single crystal including such kind of single crystal supporting apparatus (Japanese Patent Publication No.515/95), and in the figure, reference numeral 51 represents a crucible.
The bottom, almost cylindrical crucible 51 is charged with a melt 53 of a material for forming a single crystal. A pulling wire 41a is suspended above the crucible 51, and is caused to move up and down and to rotate in the direction shown by the arrow A by a wire actuator 47. To the lower end portion of the pulling wire 41a, a bar-shaped pulling axis 41b is joined, around which a pipe-shaped rotor 42 is arranged by a bearing 42a so as to be rotatable. To the upper portion of the pulling axis 41b, a motor 43 is fitted, and the motor 43 is connected to the upper portion of the rotor 42 and is electrically joined to a receiving portion 43a. When a signal transmitted from the outside of a chamber (not shown) of an apparatus for pulling a single crystal 40 using a transmitting means (not shown) is received by the receiving portion 43a, the motor 43 is driven so as to rotate the rotor 42 in the direction shown by the arrow B or C. On the outer regions of the rotor 42, an external thread portion 42b is formed, to which a disc-shaped supporting portion 44a is screwed. From the lower portion of the supporting portion 44a, a cylindrical grasping holder 44b is extended, and on the inner surface thereof, guide slots 44c are vertically formed so as to face each other. In the lower portion of the grasping holder 44b, plural notches 44d are formed, in which pawls 44e are pivoted so as to be rotatable. At the lower end portion of the grasping holder 44b, a stopper portion 44f is formed. Accordingly, the pawls 44e can turn upward, while they are inhibited from turning downward by the stopper portion 44f. A grasping means 44 includes the supporting portion 44a, grasping holder 44b, pawls 44e, stopper portion 44f, and associated parts. On the other hand, a holder 41c is connected to the lower end portion of the pulling axis 41b, and a flange 45 is fixed to the outer regions of the holder 41c. The outer regions of the flange 45 are fitted in the guide slots 44c so as to slide up and down. When the rotor 42 rotates, the grasping means 44 moves up and down as the rotation thereof is restricted. A single crystal supporting apparatus 40a includes the pulling axis 41b, rotor 42, motor 43, grasping means 44, flange 45, and associated parts. At the lower end portion of the holder 41c, a seed crystal 41d is held.
In pulling a single crystal 46 using the apparatus for pulling a single crystal 40 having the above construction, the motor 43 is driven in the normal direction of rotation so as to rotate the rotor 42 (for example, in the direction of the arrow B) and to transfer upward the grasping means 44 which is screwed thereto. The wire actuator 47 is driven so as to transfer the holder 41c downward through the pulling wire 41a and pulling axis 41b to dip the seed crystal 41d into the melt 53. The wire actuator 47 is driven so as to pull the pulling wire 41a with rotation at a relatively high speed, leading to the formation of a neck 46a. By pulling the pulling wire 41a away from the melt at a gradually decreased speed, an enlarged portion 46b having a large diameter is formed under the neck 46a. By pulling the pulling wire 41a at a relatively high speed again, a reduced portion 46c having a smaller diameter than the enlarged portion 46b is formed under the enlarged portion 46b. By pulling the pulling wire 41a at a gradually decreased speed, a shoulder portion 46d is formed under the reduced portion 46c. At that time, by transmitting a signal to the receiving portion 43a using the above transmitting means, the motor 43 is driven in the reverse direction of rotation so as to rotate the rotor 42 in the direction of the arrow C to transfer the grasping means 44 downward. When the pawls 44e touch the enlarged portion 46b, the pawls 44e are caused to turn relatively upward so as to avoid the enlarged portion 46b and are fitted to the reduced portion 46c. When the pulling wire 41a is pulled at a prescribed speed after the motor 43 is stopped, a main body 46e having a prescribed diameter is formed under the shoulder portion 46d where the reduced portion 46c is held by the single crystal supporting apparatus 40a.
To separate the grown single crystal 46 from the melt 53, the pulling wire 41a is pulled at a relatively high speed so as to gradually reduce the crystal diameter to form an inverse conical crystal tail (not shown), and the single crystal 46 is separated from the melt 53 at a point of time when the single crystal 46 has a sufficiently small diameter to prevent dislocation from propagating to a product portion even if the dislocation occurs. This step has been adopted in order to prevent the dislocation from being caused by a thermal shock and from propagating to the main body 46e which forms the product portion.
The above apparatus for pulling a single crystal 40 makes it possible to pull an upsized single crystal 46. However, when the diameter is controlled using the optical measuring means 19 in the same manner as in the case using the conventional apparatus for pulling a single crystal 30, the growth interface is hidden by the enlarged portion 46b in the formation of the reduced portion 46c, so that the fusion ring cannot be observed. Therefore, it is difficult to control the diameter of the single crystal 46 using the optical measuring means 19 in the formation of the reduced portion 46c, and, thus, it is difficult to form the reduced portion 46c having a prescribed shape. Since it is difficult to control the shape of the reduced portion 46c, it is difficult to cope with changes in the conditions such as the temperature of the melt 53, so that dislocation is easily induced to the reduced portion 46c.
The conventional apparatus for pulling a single crystal 40 (FIG. 2) by which the single crystal 46 is pulled as the reduced portion 46c is held, has a complicated structure. Since the complicated single crystal supporting apparatus 40a must be arranged in a high temperature atmosphere just above the melt 53, troubles are easily caused.
Another apparatus for pulling a single crystal has been disclosed. In this apparatus, a structure for holding a reduced portion and a pulling axis for pulling a single crystal individually ascend and descend. However, in such apparatus, it is difficult to synchronize the ascent/descent speed and the number of revolutions, and it is also difficult to certainly hold a single crystal when the central axis even slightly slips out of place.
Since the single crystal supporting apparatus 40a, forming part of the apparatus for pulling a single crystal 40, has a construction wherein the reduced portion 46c is grasped by the pawls 44e, the fitting strength is small and the pawls 44e are easy to break.